A source-grounded field effect transistor for use in a high-frequency amplifier has a capacitance between a gate for inputting a signal and a drain for outputting a signal. Due to the capacitance, the output signal may be fed back to an input side so that oscillation is generated.
The output impedance of the source-grounded field effect transistor has a characteristic of a large negative resistance when the drain voltage is lower than an operating voltage. When the drain voltage rises gradually from zero V, self-heating of the source-grounded field effect transistor may restrain the negative resistance characteristic so that oscillation may be difficult to occur. However, when the drain voltage rises rapidly, oscillation may occur before the source-grounded field effect transistor generates is heated. In order to restrain oscillation, a stabilizing circuit may be added to a high-frequency amplifier.
FIG. 4 is a circuit diagram illustrating a high-frequency amplifier including a stabilizing circuit of a prior art. As illustrated in FIG. 4, in the high-frequency amplifier according to the prior art, a stabilizing circuit 402 is connected to a gate of a field effect transistor 401. The stabilizing circuit 402 is provided with a resistor and a capacitor serially-connected with each other, and is grounded
The value of the resistor is selected so as to suppress destabilization in a low voltage area when a drain voltage of a source-grounded field effect transistor rises from zero V to an operating voltage, e.g., 10V.
FIG. 5 is a graph illustrating a difference in gain between a case where the drain voltage is a low voltage and a case where the drain voltage is an operating voltage, with regard to the prior art. In FIG. 5, the vertical axis indicates a maximum available gain, and the lateral axis indicates a frequency. In FIG. 5, a curve 501 indicates a case where the drain voltage is 2V, and a curve 502 indicates a case where the drain voltage is 10V that is the operating voltage.
FIG. 6 is a graph illustrating a difference in stability factor between a case where the drain voltage is the low voltage and a case where the drain voltage is the operating voltage, with regard to the prior art. In FIG. 6, the vertical axis indicates a stability factor, and the lateral axis indicates a frequency. In FIG. 6, a curve 601 indicates a case where the drain voltage is 10V that is the operating voltage, and a curve 602 indicates a case where the drain voltage is 2V.
As illustrated in FIG. 6, when a resistance value of the stabilizing circuit 402 is selected so that the stability factor exceeds 1 (one) at a drain voltage of 2V that is the low voltage, the maximum available gain decreases in the case where the drain voltage is 10V that is the operating voltage, as compared with the case where the drain voltage is 2V, according to FIG. 5.
According to the prior art, in selecting the resistance value of the stabilizing circuit 402, the gain of the high-frequency amplifier is not considered to be made large for the purpose of assigning priority to stabilization of output of the high-frequency amplifier.